1. Field of the Invention
The invention is generally related to oriented strand board and, more particularly, to improving the surface characteristics of oriented strand board (e.g., appearance, water, vapor, and chemical resistance, smoothness, paintability, etc.) to enable its use in a wide variety of construction applications. The invention is generally applicable to the post fabrication treatment of other wood construction products including plywood and solid sawn lumber.
2. Background Description
Oriented strand board is commercially available from a number of companies including J. M. Huber Corporation, Georgia-Pacific Corporation, Louisiana-Pacific, and a number of other sources. This material has multiple layers of wood "flakes" or "strands" bonded together by a binding material such as phenol formaldehyde resin or isocyonate resin together with sizing agents such as paraffinic waxes. The flakes are made by cutting thin slices with a knife edge parallel to the length of a debarked log. The flakes are typically 0.01 to 0.5 inches thick, although thinner and thicker flakes can be used in some applications, and are typically, less than one inch to several inches long and less than one inch to a few inches wide. The flakes typically are longer than they are wide. In the fabrication of oriented strand board, the flakes are first dried to remove water, and are then coated with a thin layer of binder and sizing agent. The coated flakes are then spread on a conveyor belt in a series of alternating layers, where one layer will have the flakes oriented generally in line with the conveyor belt, and the succeeding layer of flakes oriented generally perpendicular to the conveyor belt, such that alternating layers have flakes oriented generally perpendicular to one another. The word "strand" is used to signify the cellulosic fibers which make up the wood, and, because the grain of the wood runs the length of the wood flake, the "strands" in the oriented strand board are oriented generally perpendicular to each other in alternating layers. The layers of oriented "strands" or "flakes" are finally subjected to heat and pressure to fuse the strands and binder together. The resulting product is then cut to size and shipped. Typically, the resin and sizing agent comprise less than 10% by weight of the oriented strand board product.
The fabrication of oriented strand board is described in U.S. Pat. No. 5,525,394 to Clarke et al., and that patent is herein incorporated by reference. Oriented strand board has been used in sheathing walls, wooden I-beam structural supports, and in roofs and floors where strength, light weight, ease of nailing and dimensional stability under varying moisture conditions are the most important attributes. Oriented strand board is sold at a substantial discount compared to structural grades of soft plywood.
Oriented strand board has met with some resistance to commercial acceptance in certain applications where moisture resistance or durability is premium or where smoother surfaces are premium. For instance, in the manufacture of concrete forms, the wood material is subjected to water and chemical entities which can degrade the board, particularly at the edges. Likewise, edge delamination problems may occur in flooring applications where the board is subjected to pooled water for extended periods of time. Moreover, the coarse surface of oriented strand board can have an adverse aesthetic impact in walls, signage and other applications where the appearance of the material can be discerned after application of paint or other surface treatments.
Several techniques have been developed to address these concerns. Fore example, U.S. Pat. No. 5,525,394 to Clarke describes an oriented strand board structure which has progressively smaller strands towards its upper surface, and a dry process fiberboard overlay layer on its uppermost surface. Clarke explains that this lay up enables the top surface to be embossed with precision. Of interest, the Clarke process has found application in the commercial production of oriented strand board products which have been embossed to resemble a natural "wood grain" finish. U.S. Pat. No. 5,635,248 to Hsu et al. describes a process for producing a smooth hard finish on products such as oriented strand board. In Hsu, a foamed polymerized latex emulsion is applied to the surface and dried. After drying, the emulsion is crushed, and then cured to form the coating, with post-cure heat treatments being found to improve the hardness of the coating. U.S. Pat. No. 5,554,429 to Iwata et al. discloses an oriented strand board flooring material which is indicated to have significant moisture resistance. In Iwata, the oriented strand board is fabricated with the surface layers having strands with longer average length values and wider average width values than the centrally located layers. Iwata uses a foaming urethane resin and a non-foaming aqueous emulsion-type phenol resin in combination to join the wood strips together. Iwata also contemplates attaching a decorative sheet of material (e.g., oak veneer) to the oriented strand board surface using an aqueous polymeric isocyanate adhesive, and subsequently overcoating the decorative sheet with a polyurethane, thus producing high gloss, decorative, wood flooring which has the appearance of oak. The complete contents of each of these patents is herein incorporated by reference.
A significant disadvantage of each of the above-discussed surface treatments is that they involve using additional materials as the surface layer, and therefore result in increased costs. What is needed is a low-cost and effective surface treatment which improves the surface characteristics of oriented strand board.